A typical railroad freight car includes a car body supported on a pair of wheeled trucks which are confined to roll on rails or tracks. Each truck includes a bolster extending essentially transversely of the car body longitudinal centerline. In the preponderance of freight cars, a pivotal connection is established between the bolster and railcar body by center bearing plates and bowls transversely centered on the car body underframe and the truck bolster. Accordingly, the truck is permitted to pivot on the center bearing plates under the car body. As the railcar moves between locations, the car body also tends to adversely roll from side to side.
Attempts have been made to control the adverse roll of the railcar body through use of side bearings positioned on the truck bolster outwardly of the center bearing plates. A “gap style” side bearing has been known to be used on slower moving tank/hopper railcars. Conventional “gap style” side bearings include a metal, i.e. steel, block or pad accommodated within an elongated open top pocket or recess defined on the truck bolster. An elongated and upstanding housing or cage, integrally formed with or secured, as by welding or the like, to an upper surface on the truck bolster defines the open top recess and inhibits sliding movement of the metal block relative to the bolster. As is known, a gap or vertical space is usually present between the upper surface of the “gap style” side bearing and the underside of the railcar body.
Other conventional “gap style” side bearings have included roller bearings carried for rolling movements within the elongated housing or carrier mounted on the upper surface of the railcar bolster. The roller extends above an uppermost extent of the housing or carrier and engages with an underside of the railcar body. Such side bearings are able to support the railcar body with respect to the bolster while at the same time permitting the bolster, and therefore the truck, freedom to rotate with respect to the car body as is necessary to accommodate normal truck movements along both straight and curved track.
Under certain dynamic conditions, coupled with lateral track irregularities, the railcar truck also tends to adversely oscillate or “hunt” in a yaw-like manner beneath the car body. The coned wheels of each truck travel a sinuous path along a tangent or straight track as they seek a centered position under the steering influence of the wheel conicity. As a result of such cyclic yawing, “hunting” can occur as the yawing becomes unstable due to lateral resonance developed between the car body and truck. Excessive “hunting” can result in premature wear of the wheeled truck components including the wheels, bolsters, and related equipment. Hunting can also furthermore cause damage to the lading being transported in the car body.
Track speeds of rail stock, including tank/hopper cars, continue to increase. Increased rail speeds translate into corresponding increases in the amount of hunting movements of the wheeled trucks. “Gap style” or those side bearings including roller bearings simply cannot and do not limit hunting movements of the wheeled trucks. As such, the truck components including the wheels, bolsters, and related equipment tend to experience premature wear.
The art has also contemplated constant contact side bearings for railcars. Constant contact railcar side bearings not only support a railcar body with respect to the bolster during relative rotational movements therebetween but additionally serve to dissipate energy through frictional engagement between the underside of the railcar body and a bearing element thereby limiting destructive truck hunting movements. Constant contact side bearings typically include a housing assembly including a base or housing and a cap. The housing usually has a cup-like configuration and includes at least two apertured flanges, extending in opposed radial directions relative to each other, permitting the housing to be fastened to the bolster. In one form, the cap is biased from the housing and includes an upper surface for contacting and rubbing against a car body underside. The cap must be free to vertically move relative to the side bearing housing.
Such constant contact side bearings furthermore include a spring. The purpose of such spring is to absorb, dissipate, and return energy imparted thereto during a work cycle of the side bearing assembly and resiliently position the upper surface of the cap, under a preload force, into frictional contact with the car body underframe. The spring for such side bearings can comprise either spring loaded steel elements or elastomeric blocks or a combination of both operably positioned within a cavity defined by the side bearing housing and the cap. An elastomeric block which has been found particularly beneficial is marketed and sold by the Assignee of the present invention under the tradename “TecsPak.” As will be appreciated, however, such an elastomeric block, by itself, lacks longitudinal stiffness and, thus, requires surrounding housing structure to provide added support and stiffness thereto.
There are several challenges presented in connection with the design of a constant contact side bearing assembly. First, and during the course of operation, clearance between sidewalls on the housing and cap of a constant contact side bearing housing assembly tend to become enlarged due to abrasion and wear. Such wear is a critical detractor to side bearing assembly performance. That is, any gap or space between the sidewalls on the housing and cap of the side bearing assembly adversely permits longitudinal or horizontal shifting movements of the cap relative to the housing thereby reducing the energy absorption capability for the side bearing assembly—a critical operating criteria for the side bearing assembly. Of course, if the gap or space between the housing and cap of the side bearing assembly reaches a critical limit, the side bearing assembly is no longer useful and can be condemned.
During operation of the railcar side bearing assembly, and while controlling the clearance or gap between the cap and housing of the side bearing assembly so as to limit horizontal shifting movements of the cap relative to the housing remains advantageous, the cap must remain able to vertically reciprocate relative to the housing. As will be appreciated, if the cap cannot vertically reciprocate during operation of the side bearing assembly, the primary purpose and function of the constant contact side bearing assembly will be lost.
Designing a side bearing assembly having a multipiece cap for controlling the gap or space between the cap and wall structure on the housing and which is biased into contact with an underside of the railcar body is also known in the art. Although beneficial in limiting the clearance or gap between the cap and housing, designing a constant contact side bearing assembly with a mulitpiece cap introduces other design problems and challenges. For example, the multipiece cap members tend to vertically separate as the railcar rolls from side-to-side. That is, after the car body rolls in a first direction, the cap members of one side bearing assembly are allowed to vertically separate relative to each other. When the railcar body again rolls in an opposite direction, the vertically separated cap members of the one side bearing assembly are vertically crushed against each other by the underside of the car body. Especially when the cap members are formed from a non-metal materials, this continuous rolling action of the car body can have an adverse affect on the cap members. Of course, any cracking or sticking of the cap members relative to the housing can and often does result in condemnation of the side bearing assembly. The ability to limit vertical separation of the cap members relative to each other, however, is complicated when considering the requirement such cap members must also maintain their ability to horizontal shift or slide relative to each other so as to limit or reduce the clearance between the cap members and outstanding wall structure on the side bearing assembly housing.
Another design challenge involved with those constant contact side bearings using an elastomeric spring relates to the buildup of heat in proximity to the elastomeric spring. During operation of the railcar, frictional contact between the railcar body and the side bearing assembly results in the development of heat buildup. Unless such heat buildup can be controlled, the elastomeric spring will tend to soften and deform, thus, adversely affecting the operable performance of the constant contact side bearing assembly.
The frictional sliding relationship between the side bearing assembly and the related railcar component can create temperatures within the side bearing assembly that can exceed the heat deflection temperature of the elastomeric spring thus causing the elastomeric spring to deform. As used herein and throughout, the term “heat deflection temperature” means and refers to a temperature level at the which the elastomeric spring, regardless of its composition, tends to soften and deform. Deformation of the elastomeric spring can significantly reduce the ability of the elastomeric spring to apply a proper preload force and, thus, decreases vertical suspension characteristics of the side bearing assembly which, in turn, results in enhanced hunting of the wheeled truck. Enhanced hunting and/or unstable cyclic yawing of the truck increases the resultant translation/oscillation of the railcar leading to a further increase in the heat buildup and further deterioration of the elastomeric spring.
Thus, there is a continuing need and desire for a railcar constant contact side bearing assembly including a multipiece cap design which allows the cap members to horizontally slide or shift relative to each other whereby optimizing energy absorption and related performance criteria for the side bearing assembly while maintaining vertical reciprocity of the cap members relative to the housing and which limits vertical separation of the cap members relative to each other